The present invention relates to a method of and an apparatus for modulating a beam of light. More particularly, this invention relates to a light modulator structure for producing high-contrast operation using zero-order light.
Light modulators are used in many telecommunications applications including optical switching. Light modulators, and in particular grating light valve(trademark) light modulators, utilize first order light to obtain higher-contrast operation. A conventional grating light valve(trademark) light modulator includes a series of elongated, reflective ribbons arranged adjacently and in parallel. When the ribbons lie in an un-deflected, or flat, state, an incident beam of light reflects off the grating light valve(trademark) light modulator as a mirror. If alternating ribbons are pulled down, or deflected, then the incident light diffracts. In operation, a through-state is considered to be when the alternating ribbons are deflected by a predetermined distance, thereby obtaining maximum diffraction of the incident light, and the diffracted first order light is collected. Inefficiencies arise since the first order light is not the only diffracted light in the deflected state. Higher orders of light are also produced including second order, third order, etc. These higher orders are not collected and are therefore wasted. This reduces efficiency.
Problems arise in association with the collection process of the first order light When the incident light is diffracted in the through-state, different wavelengths diffact at different angles. Larger wavelengths have larger diffraction angles. As such, any wavelength combiner used for collecting the first order light must be sufficiently large to account for the varying wavelength diffraction angles. A wavelength combiner is also called a wavelength multiplexer, examples of which include a dichroic filter, a diffraction grating, and an array waveguide. Unfortunately, the larger the combiner, the less efficient is the collection process and the lower the contrast ratio provided. Further, since the first order light includes a plus and minus component, two such filters are necessary to collect each the plus and minus first order light.
Design of the optical system must not only account for collecting of the plus and minus first order light, but also must isolate the first order light from the higher order light. Since the different wavelengths diffract at different angles, the optical system must ensure sufficient discrimination of zero and first order diffraction of all wavelengths. Considering the isolation and collection constraints of such a system, the optical design considerations using a conventional grating light valve(trademark) light modulator are substantial.
Additionally, high-contrast operation is essential for performing optical switching where 30-40 dB of contrast is necessary. However, conventional zero and first order operation often does not produce high enough contrast needed for optical switching.
What is needed is a spatial light modulator and associated optical system that is easier to design and implement. What is also needed is a spatial light modulator and associated optical system with increased contrast and improved efficiency.
An optical system of the present invention provides high-contrast operation by collecting zero order light. The optical system comprises a light modulator and a collector. The light modulator is preferably a grating light valve(trademark) light modulator including a plurality of elements selectively operable in a first mode and a second mode, wherein a gap between adjacent elements is equal to or less than a wavelength of an incident light beam. The plurality of elements in the first mode reflect light along a return path, where the plurality of elements in the second mode direct light away from the return path. The collector is coupled to the light modulator to collect zero order light along the return path while the plurality of elements are in the first mode and in the second mode.
The collector comprises Schlieren optics and an optical train. The Schlieren optics pass the zero order light. The optical train directs the zero order light to an optical fiber, a display device, a thermal printer or any other appropriate application.
The first mode comprises a reflection mode in which the plurality of elements reflect the incident light beam as a plane mirror. The second mode comprises a diffraction mode in which the plurality of elements comprise a first group of elongated elements interdigitated with a second group of elongated elements. A height difference between the first elongated elements and the second elongated elements causes the incident light beam to diffract away from the return path.